As concrete is being laid under water, the cement paste portion is washed away from a part of the concrete mix upon contact with water and the concrete has a nonuniform composition, either as the concrete mix is falling through the water or as the concrete mix is spreading in a lateral direction. As a result, the strength of the concrete mix is reduced, and the separated cement paste is diffused into the surrounding water rendering it turbid. In order to resolve these problems, in a structural material mixture for underwater structures, there is disclosed a technique of adding to concrete a cellulose derivative or polyacrylamide, i.e., an acrylamide homopolymer as an admixture (DE-O-No. 2541747). However, when such an admixture is used, the obtained concrete mix has a delayed setting time and a lower strength, particularly initially, than a concrete mix containing no such agent.
A concrete containing a polyacrylamide partial hydrolysate compound as disclosed in Japanese Patent Disclosure No. 59-54656 has excellent characteristics as an underwater concrete in that it does not have a delayed setting time or a low initial strength. However, this polymer compound has a strong cohesive force with respect to cement as in the case of an acrylamide homopolymer. Therefore, the concrete mix tends to harden and is difficult to mix. When mixing of the concrete mix is difficult, mixing to obtain a uniform composition is time-consuming. In addition, the load on a mixer is increased, resulting in an impractical concrete composition.
A conventional method for adding a water soluble vinyl polymer to a cement composition is known. For example, U.S. Pat. No. 4015991 discloses as a water soluble vinyl polymer a hydrolysate of a copolymer containing 2-acrylamido-2-methylpropane-sulfonic acid units. U.S. Pat. No. 4340525 discloses a copolymer containing vinyl sulfonic acid units as a water soluble vinyl polymer. However, such water soluble vinyl polymers are used in a cementing technique to prevent water in the cement composition from being absorbed in a porous stratum. Therefore, even if such a water soluble vinyl polymer is used in a technique for laying concrete under water, the separation amount of cement continues to be large and setting time of the concrete is still delayed. Therefore, when such a water soluble vinyl polymer is used in underwater concrete, the concrete mix cannot be prevented from deteriorating under water. Furthermore, since the strength of the concrete mix is also considerably reduced, a water soluble vinyl polymer cannot be used as an underwater concrete admixture.